SIMPLE PROCEDURE FOR SIMULTANEOUS RECOVERY OF DISSOLVED INORGANIC ANDORGANIC NITROGEN IN N-15-TRACER EXPERIMENTS AND IMPROVING THE ISOTOPIC MASS-BALANCE

We developed a simple and reliable method which allows simultaneous is otope-ratio analysis of inorganic (DIN) and organic (DON) forms of nit rogen extracted from seawater. All forms of nitrogen under analysis ar e converted to ammonium, by diffusion with magnesium oxide, prior to c ollection on glass-fiber filters appropriate for mass spectrometric as say of N-15. Oxidized DIN forms (nitrate, nitrite) are reduced to ammo nium in the presence of Devarda alloy. Conversion of DON to ammonium i s performed by wet oxidation using potassium persulfate and subsequent reduction of the nitrate formed. Recovery tests, both for total nitro gen and N-15 content, showed that this procedure is suitable for appli cation in (DIN)-N-15-isotope dilution experiments and DON-release stud ies. Recovery of total nitrogen from DIN and DON was nearly complete ( 94 to 97%). The variability in the experimental determination of N-15 abundance was < 2% and < 4% for DIN and DON, respectively. We used the method to balance the N-15 budget in nitrate and ammonium uptake expe riments conducted in an oligotrophic area (tropical North Atlantic) by including, in addition to the substrate (DIN) and biomass (PON) pool, the DON pool. However, the use of glass-fiber filters (GF/F) for the collection of particulate matter produced a significant artifact, i.e. a large amount of small particles (< 0.7 mu m, PON<GF/F; prochlorophy tes and/or bacteria) passed through these filters and were recovered t ogether with the DON in a combined pool. While inclusion of this combi ned pool led virtually to a complete accounting for the N-15 label (99 %) in all samples for nitrate uptake and in those for ammonium uptake incubated for < 8 h, no mass balance was achieved during ammonium upta ke lasting 10 to 24 h. We suggest that the N-15 that was still missing (13%) resulted mainly from bottle containment effects such as ammoniu m-ion adsorption and/or PON adherence onto incubation bottle walls. Tr ansfer of N-15 label to the combined pool (nitrate experiment) and to the DON and PON<GF/F pools (ammonium experiment) represented up to 41, 38 and 20% of the total N-15 taken up as DIN, respectively, and depen ded strongly upon the length of incubation. Failure to take these path ways of the missing N-15 into account during traditional N-15 uptake e xperiments involves risk of substantially underestimating new and rege nerated production, at least in oligotrophic areas. The latter fact ha s considerable significance in the design of future N-15 tracer method ologies.